Multiport Tunable Optical Filters

ABSTRACT

A tunable multiport optical filter includes various types of arrays of optical ports. The tunable filter also includes a light dispersion element (e.g., a grating) and a reflective beam steering element (e.g., a tilting mirror). An optical signal exits an optical (input) port, is dispersed by the light dispersion element, reflects off the reflective beam steering element back to the light dispersion element, and on to another optical (output) port. The reflective beam steering element can be steered such that a wavelength portion of the dispersed optical signal can be coupled to the optical output port. For example, the input optical signal may be a wavelength division multiplexed signal carrying multiple channels on different wavelengths, and the tunable multiport optical filter directs one of the channels to the output optical port. Additionally, the tunable filter may be incorporated into a device act as a wavelength reference.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is related to U.S. application Ser. No. 12/804,627filed on Jul. 26, 2010, U.S. application Ser. No. 12/927,066 filed onNov. 5, 2010, and U.S. application Ser. No. 13/226,275 filed on Sep. 6,2011. The subject matter of all of the foregoing are incorporated hereinby reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to multiport tunable optical filters,for example as may be used for optical channel monitoring.

2. Description of the Related Art

Optical devices that can be tuned to select one or more wavelengths froma wider wavelength spectrum are known as tunable optical filters. Theyare used in a variety of optical systems, e.g., wavelength divisionmultiplexed (WDM) systems. In WDM systems that operate typically overwavelength bands of tens of nanometers, tunable optical filters are usedfor optical performance monitoring (OPM) to ensure that signal power,signal wavelength, and/or optical signal to noise ratios (OSNR) arewithin specified limits. Other applications for tunable optical filtersinclude optical noise filtering, noise suppression, the actualwavelength division demultiplexing function, and optical routing.

Dense wavelength division multiplexed (DWDM) systems have manywavelength channels operating across a wide optical spectrum.Additionally, many tunable optical filters are limited in terms of howtheir fiber input ports and fiber output ports are configured and thenumber of those ports. For example, a large linear array of ports mayincrease optical aberrations as the ports move away from the opticalaxis of the system, thus limiting the size of the array. Additionally,adding ports in this manner increases the form factor (e.g., size) ofthe device.

Additionally, with the introduction of DWDM systems operating on aflexible grid, a channel monitoring device may be required to report thepower in a given frequency range, identified by a start and a stopfrequency. Additionally, the system may be required to be able to locatethese frequencies with an accuracy of 1 GHz. This is not readily donethrough calibration, as small changes in the optical path over thelifetime of the device can easily account for shifts that easily exceed1 GHz. Moreover, the accuracy of 1 GHz is difficult to achieve even witha bench-top instrument.

SUMMARY OF THE INVENTION

In at least one embodiment, a tunable multiport optical filter includesa linear array of optical input ports extending along an x-direction andan optical output port. The optical input ports are positioned on aregular grid in the x-direction, where the regular grid is characterizedby a pitch. For example, adjacent optical input ports may be separatedby N×pitch where N is an integer (most often 1). The optical output portis positioned collinearly with the optical input ports but is off theregular grid. For example, the optical output port may be separated fromthe nearest optical input ports by (N+0.5)×pitch. The tunable multiportoptical filter also includes a light dispersion element (e.g., agrating) and a reflective beam steering element (e.g., a tiltingmirror). An optical signal exits an optical input port, is dispersed inthe y-direction (i.e., a direction different from the x-direction) bythe light dispersion element, reflects off the reflective beam steeringelement back to the light dispersion element, and on to the opticaloutput port. The reflective beam steering element can be steered suchthat any wavelength portion of the dispersed optical signal from anyoptical input port can be coupled to the optical output port. Forexample, the input optical signal may be a wavelength divisionmultiplexed signal carrying multiple channels centered on differentwavelengths, and the tunable multiport optical filter directs thechannels to the output optical port.

In another aspect, a tunable multiport optical filter includes atwo-dimensional array of optical ports, which includes optical inputports and their corresponding optical output ports. The two-dimensionalarray extends along both an x-direction and a y-direction that is notparallel to the x-direction. The tunable multiport optical filter alsoincludes a light dispersion element and a reflective beam steeringelement that operate similarly as described above. That is, the lightdispersion element disperses the optical signals along the y-direction,and the reflective beam steering element can be steered to couple adesired wavelength portion of the dispersed optical signal to thecorresponding optical output port. The two-dimensional array is arrangedto reduce coupling into other optical ports.

In another aspect, an optical channel monitoring (OCM) device usestunable filters for example as described above. The OCM device isself-calibrating in that a reference spectrum is used to calibrate thetunable filter. In one approach, the reference spectrum is generated byan independent source, for example a broadband LED source coupled to awavelength filter. In another approach, part of the incoming signal istapped to produce the reference spectrum, for example by passing thetapped portion through an etalon filter. In certain embodiments, thereference spectrum has well-defined reference points (e.g., peakwavelengths) located outside the operating wavelength range for datatransmission. In other embodiments, the reference spectrum iswell-defined within the operating wavelength range.

Other aspects of the disclosure include methods and systemscorresponding to the devices described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention has other advantages and features which will be morereadily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view showing a first embodiment of a tunablemultiport optical filter;

FIG. 2 are diagrams illustrating the operation of the tunable multiportoptical filter of FIG. 1, showing ray diagrams for x- and y-crosssections of the device;

FIG. 3 is a schematic illustration of the operation of an exampletunable multiport optical filter that may be configured for 8×1operation;

FIG. 4 is a perspective view of an example tunable multiport opticalfilter including two subarrays of ports;

FIG. 5 illustrates an example multiple fiber port array, shown in thex-y plane;

FIG. 6 is a block diagram of an example process to increase theresolution of a tunable multiport optical filter;

FIG. 7 is a block diagram of an example optical channel monitoringdevice configured to self-calibrate; and

FIG. 8 is a block diagram of an example optical channel monitoringdevice configured to self-calibrate using an optical signal from anoutput port of a tunable filter to generate a reference spectrum.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation. Thefigures and the following description relate to preferred embodiments byway of illustration only. It should be noted that from the followingdiscussion, alternative embodiments of the structures and methodsdisclosed herein will be readily recognized as viable alternatives thatmay be employed without departing from the principles of what isclaimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first embodiment of a tunable multiport optical filterwith an array of input and optical fibers shown at 11, and withcollimating lens 14, dispersive element 15, and tuning mirror 16. Thetuning mirror rotates around axis 17. It should be understood that thisfigure (and subsequent figures) is not drawn to scale. Optical elementsare located and spaced according to their functions and properties asknown in the art. The description herein uses x- and y-axial notationsfor directions around the z-axis, which is the direction of lightpropagation through the device. In the following examples, the x- andy-axes are orthogonal, but this is not required. The y-axis could be atangles other than 90 degrees relative to the x-axis. Reference herein tothe x-plane or the y-plane will be understood to mean the x-z or y-zplanes. Reference in the figures to the x-axis cross section or they-axis cross section is intended to mean a view of the x-z plane or they-z plane respectively.

The embodiments shown are described in the context of opticalperformance monitoring (“OPM”) applications. However, it should beunderstood that the basic devices described herein are also useful aswavelength selective devices for routing selected WDM channels, as wellas for other applications.

The specific optics as represented by ray optics, for the embodiment ofFIG. 1, are illustrated in FIG. 2. The input fiber array 11 is composedby 8 fibers which are tightly aligned in parallel, i.e., the opticalfibers have minimal spacing and are aligned with the centers of theoptical fibers on a common axis, as shown the x-axis. The signal beam tobe monitored, typically a tapped portion of the signal from one channelof the network, is coupled to input fiber 12. It passes throughcollimating lens 14 to collimate the Gaussian input beam to collimatedlight with suitable diameter. The collimated beam is incident ontodispersion element 15. In the x-axis cross section (top portion of FIG.2) the beam is not dispersed. In the y-axis cross section (bottomportion of FIG. 2) the light beam from dispersion element 15 isdispersed into the wavelength components of the signal beam. Thewavelength components 17 a are angularly dispersed from the dispersionelement in different directions depending on the wavelength of the beam.Tuning mirror 16 is positioned as shown to intersect the dispersed beam,and is rotatable about the x-axis and optionally also the y-axis. Thelatter rotation allows control and maximization of the output coupling.

The optical fibers are shown only schematically in the figures.Typically they will be standard single mode fibers with a claddingdiameter of 125 microns and a core diameter of 10 microns or less. Inthe portion of the array shown, i.e., the portion addressed by thewavelength selection elements, the optical fibers are stripped of theusual polymer coating. This allows greater precision in the array,producing, in many cases, a predictable spacing between cores of thefibers. Recognizing that a variety of options in the format of the arraymay be desirable, as will be discussed in greater detail below, opticalfibers with sizes other than the conventional 125 microns may be useful.For example, cladding diameters of 50, 62.5, 250, may be used toadvantage to vary the overall aperture (size) of the array. It isexpected that small aperture arrays may be most cost effective. Opticalwaveguide array could also be used to implement the same function.

Mirror 16 is rotatable about the x-axis into one of many positions. Inthe example shown in FIG. 2, only one of the beam components (wavelengthchannels), in this case components represented by arrow 17 b, is normalto the mirror 16. That beam component is reflected back along a pathrepresented by the dashed line. Other beam components, like the twoshown in the y-axis cross section of FIG. 2, will “walk-off” the mirror16. The selected beam component, 17 b, is dispersed by element 15 by thesame angle as before and propagates to output fiber 13. The intensity ofbeam component 17 b is measured by photodiode 21, coupled as shown tothe output fiber 13. Input optical fiber 12 is shown in this view justto orient the viewer to the fact that the optical fibers in the arrayare stacked in the y-direction.

When mirror 16 is rotated about the x-axis, another beam component(wavelength channel) will be normal to the mirror 16 and will beselectively reflected back through output fiber 13 and its propertiesmeasured. In this manner, the wavelength spectrum of the input beam tooptical fiber 12 may be scanned and the properties of all of its beamcomponents can be measured.

Thus the device achieves wavelength selection and provides an opticalfilter. The wavelength of the filter is tuned by the rotationalorientation of mirror 16.

It should be noted that a similar result can be obtained if the axis ofthe dispersive element is rotated by 90 degrees and the mirror is tiltedin the same axis that the beam is dispersed. In this configuration thelight beam from the grating is dispersed into the wavelength componentsof the signal beam along the same axis of the fiber array, and there issome likelihood that the spectra from a fiber port will overlap with anadjacent or non-adjacent fiber port. The wavelength components which arediffracted from the dispersion element can be distinguished byincreasing the separation of the fiber ports, although this will requirea large optical aperture. To obtain satisfactory performance fiber portseparation would be increased to three or more times larger than theseparation required when the axis of the dispersive element isorthogonal to the fiber array.

It should be understood that a function of the rotating mirror 16 is toselect a wavelength component of the incident beam and return it to afixed position, in the case of the arrangement of FIG. 2, to outputoptical fiber 13/detector 21. An equivalent result may be achieved usinga refractive beam steering element, and tilting or translating therefractive element to select a given wavelength and steer it to a fixedoutput/detector. A simple implementation of this is a lens or a flat orwedged transparent plate. The output/detector is this case is located ona side of the plate opposite to the input optical fiber. In thisdescription, reference to a mirror or to a beam steering element shouldbe construed as referring to and including equivalent structures such asthose just mentioned.

It should be recognized that the optical paths in the x-axis crosssection of FIG. 2 are unchanged by the tilt of mirror 16 in the y-zplane. This is due to the fact that lens 14 focuses all input beams onthe axis of rotation of the mirror. The reflecting surface along thetilt axis remains essentially fixed when the mirror is tilted.

The orientation of the mirror may be changed by an actuator or severalactuators. The mirror may comprise a micro electro-mechanical system(MEMS), or comprise a discrete mirror driven by motors or any othertypes or actuators. The tilt of the mirror may be changed in one axis,or more than one axis.

Another WDM channel may be input as an input beam to optical fiber 18.The output of the beam components from this channel are directed throughoutput optical fiber 19 and measured by the associated photodetector asshown in the top portion of FIG. 2.

It should be appreciated by those skilled in the art that, while thearray of input optical fibers, e.g., 12 and 18, and the array of outputfibers 13 and 19 are shown closely packed and precisely aligned, thedevice input optical fibers and the device output optical fibers mayhave any length and be routed in any suitable fashion to othercomponents/couplers in the system. For example, the photodetectors 21are shown as an array of elements receiving light beams directly fromthe closely packed array of output optical fibers. However, the opticalfiber 13 may route an optical signal to a photodiode non-aligned withrespect to the output array of optical fibers.

The detection device may take any of a variety of forms, and measure avariety of optical beam properties. The arrangement shown is simple anduseful for illustration. If the input beams are suitably time divisionmultiplexed, a single detection device may be used. Alternatively asingle spectrum analyzer may be used as the detection device.

In this description the optical elements are shown as separate elements.These represent functional elements. The physical elements providingthese functions may, in some cases, be combined as a single module. Forexample, a grating may have a reflective surface or an attached orintegral lens.

Additionally, in some embodiments, the mirror 16 may be rotated in adirection orthogonal to the axis of rotation shown in FIG. 1 as furtherdescribed in U.S. patent application Ser. No. 12/927,066.

In the embodiments of FIGS. 1 and 2, the device is shown with an 8-fiberarray, and can produce 4 signal paths from one input port to acorresponding output port (input port 12 to output port 13, input port18 to output port 19, etc.). All of the signal paths are tunedsimultaneously by rotating mirror 16. In this array, all of the opticalfibers are aligned in a single plane. The number of fibers may vary buttypically will be an even number so that the fibers can be paired toproduce signal paths, with one fiber acting as the input port and theother fiber in the pair acting as the output port. Note that otherpairings of the fibers may be possible within the same device. Forexample, in FIGS. 1 and 2, the input port 12 may be coupled to outputport 19 by rotating the mirror 16 around the y-axis.

In embodiments where there is only one output port, only one input portat a time can be measured. Thus the channel reporting can only beupdated every N×t_(s) seconds, where N is the number of input ports andt_(s) is the port scan time (typically in the hundreds of milliseconds).Increasing the number of input ports thus increases the time betweenupdates. While FIG. 2 only shows two input and two output beams, it maybe configured to receive four input beams and four output beams to thefour photodetectors 21 (which will be referred to as a 4×4configuration). For applications requiring faster reporting updates, the4×4 configuration may be used. Here each input port is routed to adedicated output port. Since the input and output ports are placedsymmetrically with respect to the optical axis, all outputs areilluminated simultaneously, thus allowing the scanning of each inputport to occur in parallel. The 4×4 configuration does not require anymodification to the optics, only the photodetector for each output beamand the electronics for the parallel processing of the data. Since thefilter is tuned to the same wavelength on all ports, this device canalso be used to dwell or scan over the frequency width of a singlewavelength so that the power of the corresponding channel can becontinuously monitored on all ports simultaneously.

In a standard configuration of the fiber array, the fibers are laid outwith a constant pitch. Pitch is the distance between the centers of thecores of adjacent fibers. Fibers on one side of the collimating lens(e.g., lens 14) optical axis (indicated with a dash-dot line) can beused as either input or output only. This is the case as input ports oneither side of the optical axis may receive light from another port,thus compromising the device “directivity”, i.e. the multiport deviceproperty for which light from one input port does not come out fromanother input port. Thus an 8-fiber array (e.g., as seen in FIG. 2) canbe used either for a 4×1 device, or as a 4×4 device. For the formerconfiguration only five of the eight fibers would be utilized, four asinput ports and one as an output port.

FIG. 3 is a schematic illustration of the operation of an exampletunable multiport optical filter 300 that may be configured for 8×1operation. FIG. 3 shows a linear array of optical fiber input ports 310,320, 330, 340, 350, 360, 370, and 380, and a fiber output port 390. Theinput ports are located on a regular grid in the x-direction. Theregular grid is characterized by a pitch. Thus, adjacent input ports arespaced by an integral number of pitches in the x-direction, although itcould be more than 1×pitch. For example, the spacing from port 380 to370 is 1×pitch, but the spacing from port 350 to 340 is 3×pitch. Theoutput port 390 is collinear with the input ports but is positioned offof the grid. In this example, it is at 1.5× the pitch from any adjacentport. For example, the diameter of a typical fiber is 125 μm, and thecorresponding pitch is typically 127 μm. Thus, between the output port390 and any adjacent port the distance would be 190.5 μm (1.5×127 μm),whereas the distance between input ports is N×127 μm where N is aninteger.

The spacing of the optical ports is such that when a single input portis selected to be monitored via output port 390, any signal from anon-selected input port is reflected in a manner such that there isnegligible coupling with the output port 390 or any of the other inputports. For example, in FIG. 3, the tunable multiport optical filter isconfigured to monitor an optical signal from input port 310. The filter300 adjusts the angle of the mirror 16 such that the optical signal frominput port 310 couples to output port 390. Due to the geometry of thesystem, and in particular, the spacing between the ports, whenmonitoring a particular port, any optical signals from non-monitoredports are reflected by the mirror 16 in a manner that results in littleto no coupling or cross talk to other ports (e.g., reflected signals areincident in the space between the input ports or are reflected away fromthe port array).

In other embodiments, the output port 390 may be placed anywhere in thearray, provided that it is not located on the regular grid defining thepositions of the input ports. Preferably, its distance from an adjacentinput port is (N+0.5)×pitch where N is an integer. Note that the devicemay also be used in a 4×4 configuration, for example where the top 4optical ports are optical input ports which are coupled to the bottom 4optical ports as optical output ports. In an alternate embodiment, theoptical output port 390 may be placed at other locations, for example asan outer port (bottom or top) in the array of ports.

As discussed below, as the amount of aberration generally increases withthe distance from the optical axis. The tunable multiport optical filterillustrated in FIG. 3 makes it possible to double the number of inputports without a significant increase of the distance between the fibersand the lens optical axis. Thus the optical performances of this 8-inputport device is essentially the same as the 4-input port device shown inFIGS. 1 and 2.

As the number of ports is increased, the optical aberrations alsoincrease due to the increased aperture requirement for the collimatinglens 14. Standard aspheric lenses do correct for spherical aberration onthe optical axis, however when working off axis, like in the case for amultiport tunable filter, coma aberration is also present. Withreference to FIG. 3, the coma aberration increases linearly with thedistance from the optical axis. Thus, adding more ports in the lineararray results in a rapid degradation of the optical performance.

Systems with higher port count may be implemented using the twodimensions of the lens focal plane, i.e., arranging the fibers as a2-dimensional array in the x-y plane. In this way a higher port countdoes not correspond to an increased distance of the fibers from theoptical axis. For example, an 8×8 device can be implemented still usingthe same basic size of FIG. 3 if the fibers are laid out as in FIG. 4.

FIG. 4 is a perspective view of an example tunable multiport opticalfilter 400 including an array of 16 fiber ports, which can be subdividedinto two subarrays of 8 fiber ports each. The tunable filter 400includes a first fiber port subarray 410 and a second fiber portsubarray 420 that is shifted by 0.5×pitch with respect to the first portsubarray 410. Because of the circular symmetry of lens 14, each fiber isimaged into the fiber placed symmetrically with respect to the opticalaxis. Thus, the optical signal leaving port 430 couples into the port440 and the optical signal leaving port 450 couples into port 460.

Because of the presence of the grating, the image of each fiber will bedispersed based on the spectrum of wavelengths present in the lightcoming out from the fiber. Thus, in general the image is a line ratherthan a single point. Additionally, only a small fraction of thewavelength spectrum couples into an output fiber and the remainingportion should be rejected. For typical spectral width used intelecommunication systems (about 40 nm) a person skilled in the art cancalculate that the length of the dispersed image is greater than 1 mm,i.e., much larger than the fiber core diameter (about 0.01 mm). Thus, ifthe two linear subarrays of fibers are not shifted with respect to eachother, there is the possibility that some of the rejected light couldcouple into an adjacent fiber lying to the left (or right) of theselected output fiber, thus resulting in cross talk between ports.

The half-pitch shift of the first port subarray 410 with respect to thesecond port subarray 420 minimizes cross talk between ports. With thislayout the rejected light will land in between two adjacent fibers, i.e.at about 60 micron from their cores, which ensures enough isolation tosatisfy typical specifications (about 40 dB).

In some embodiments, additional fiber port subarrays may be added thatare shifted by a fraction of the pitch distance, thus extending to portcounts greater than 8×8. In general the concept can be extended to N×Nports. For example, FIG. 5 illustrates an example port array 500 thathas four subarrays, shown in the x-y plane. As in FIG. 4, the fiber portarray in FIG. 5 is symmetric about the origin. That is, for every fiberlocated at position (x,y), there is a corresponding fiber located atposition (−x-y), and these two fibers form a signal path. For example,in FIG. 5, fiber 510 and 590 form one such pair. Array 500 may, forexample, be used for a 16×16 device. In FIG. 5, each subarray of 8fibers is shifted by 0.25×pitch so that these line images do not coupleinto unintended fibers.

For a fixed size, increasing the number of ports reduces the x offsetbetween the fiber cores and thus increases the cross talk between ports.For instance, in the N=16 example shown in FIG. 5, the x offset is0.25×pitch (as opposed to 0.5×pitch in FIG. 4). The isolation I betweenports can be estimated using the formula (in dB)

I≅4.34[d/w₀]²  (1)

where d is the x offset and w₀ is mode field radius of the fiber.

For example, assuming the N=16 configuration of FIG. 5 and typicalvalues (d=127/4=31.75 μm, w₀=5.2 μm) the calculated isolation is greaterthan 100 dB. The actual limitation is thus due to the coupling of thecladding modes into the core fundamental mode. If scattering centerssuch as micro-bending, air bubbles, etc., are minimized, the isolationcan meet the typical requirements (about 40 dB), even for fiberseparations as small as 20 μm. If the configuration of FIG. 5 isextended to contain 2 m subarrays of 8 fibers each (m=1 in FIG. 4 andm=2 in FIG. 5), then this would correspond to a 8 m×8 m array with an xoffset of 1/(2 m)×pitch. This is only one configuration and others arealso possible. For example, each subarray could contain 6, 10 or othernumbers of fibers. The fibers could be arranged in semi-regular or evennon-regular patterns. For example, in FIG. 5, subarrays 2 and 3 (the twoclosest to the x-axis) could be swapped.

The extension to two dimensions can increase the number of ports withoutrequiring a corresponding increase of device dimensions, thereby keepingthe same basic design without compromising optical performance. With theincreasing deployment of networks whose channels are more tightly spacedin wavelength, it is desirable to increase the resolution of the tunablefilter. A Gaussian beam based analysis of the system in, e.g., FIG. 3shows that the filter shape is Gaussian with a full width half maximumFWHM=0.831×λ/N, where N is the number of grating lines illuminated bythe beam and λ is the light wavelength. Increasing the number ofilluminated grating lines would then decrease the width of the filterresponse and thus increase its resolution. In some embodiments, N may beincreased by using a larger optical beam. However, this may increase thesize of the grating and thus results in larger device dimensions.Additionally, in some embodiments, the number of illuminated gratinglines can also be increased by using a grating with higher linedensities. For example, a high diffraction efficiency, polarizationindependent transmission grating with 1200 lines per millimeter or moremay be used. The use of the high density grating may result in increasedfilter resolution without significant change of the device dimensions.Moreover, in some embodiments, signal processing (e.g., deconvolution)may be used to enhance the resolution of the filter. FIG. 6 is a blockdiagram of an example process 600 to increase the resolution of atunable multiport optical filter.

In FIG. 6, a tunable filter 610, receives an input signal 605. Thetunable filter 610 may be, for example, any of the tunable filtersdiscussed with reference to FIGS. 1-5. FIG. 6 shows an example spectrumof the input signal 605 (measured by a bench top optical spectrumanalyzer). This spectrum is composed of three channels spaced apart by0.4 nm (50 GHz) from one another. The power of the middle channel is onetenth of the powers of the two side channels. The tunable filter 610outputs an optical signal, which is detected and digitized by thephotodetector and analog-to-digital converter 620. In some cases, suchas the one represented by 615, due to the resolution of the tunablefilter 610, the presence of a channel in between the two peaks is nolonger obvious. The digital signal is then passed to signal processor630. Signal processor 630 applies a deconvolution to the signal toincrease the signal resolution resulting in the processed signal 635.The peak of the middle channel is now detectable, so that its presence,as well other parameters such as power and wavelength, can be accountedfor.

With the introduction of DWDM systems operating on a flexible grid, achannel monitoring system, which comprises a tunable filter, may bedesirable to report the power in a given frequency range, identified bya start and a stop frequency. One requirement may to be able to locatethese frequencies with accuracy of 1 GHz. Embodiments discussed belowmake this possible using self-calibrating optical channel monitoring(“OCM”) devices.

FIG. 7 is a block diagram of an example OCM device 700 configured toself-calibrate. The OCM system 700 includes a light source 710, a sourcefilter 720, a tunable filter 730, a controller 740, and a photodetector760. While only a single input and a single output (with correspondingphotodetector 760) are shown, it should be understood that the inventionis applicable to an OCM device with any number of optical input portsand any number of photodetectors 760. The light source 710 is a broadband source, for example, an LED or SLED. The light source 710 generatesa source spectrum that is filtered by the source filter 720.

The source filter 720 includes at least two pass bands that are locatedjust outside the wavelength range over which the tunable filter 730 isintended to operate. For example, if the tunable filter is used in adata transmission system and the operating wavelength range for the datatransmission (i.e., for the input optical signal) is 1528.710 nm to1563.86 nm, the source filter 720 may provide pass bands centered at1522 nm and 1570 nm. The source filter 720 filters the source spectrumto produce a reference spectrum with peaks at the pass band locations.Additionally, in some embodiments, instead of being located on eitherside of the operating wavelength range, the pass bands may be located onthe same side of the operating wavelength range, for example pass bandsat 1570 nm and 1600 nm. Additionally, in some embodiments, the stabilityof these pass bands may be ensured using special packaging of the sourcefilter 720 that incorporates a temperature controller (not shown). Thelight source 710 and source filter 720 together produce the referencespectrum, but the reference spectrum may be produced by other types ofreference spectrum sources, such as precisely controlled tunable laseror one or more distributed-feedback (DFB) lasers.

The reference spectrum may be input to the tunable filter 730 eitherusing an auxiliary input port or by using a directional coupler to oneof the tunable filter ports. The tunable filter 730 may be, for example,any of the tunable filters discussed with reference to FIGS. 1-6.

The output of the tunable filter 730 may be monitored by thephotodetector 760. While only a single photodetector is shown, inembodiments with multiple outputs, each output may have a dedicatedphotodetector 760. Moreover, in some embodiments, one or more outputoptical ports of the tunable filter 730 may be replaced with acorresponding photodetector. Moreover, in some embodiments the OCMdevice 700 may include one or more electronic amplifiers (not shown)that amplify the electronic output signal produced by the photodetector760.

The controller 740 includes a memory 745 and a processor 750. Thecontroller 740 is configured to execute instructions that calibrate thetunable filter 730 using the reference spectrum. The calibration mayoccur continuously, periodically, upon receipt of a calibration command,or some combination thereof. The controller 740 calibrates the tunablefilter 730 using the reference spectrum. For example, memory 745 maycontain a lookup table that maps wavelength (or frequency) to reflectivebeam steering element (e.g., mirror 16). During a calibration process,the tunable filter 730 may scan across a range of wavelengths until atleast two peaks in the reference spectrum are detected. The controller740 records the reflective beam steering element positions for thosepeaks in memory 745. A reflective beam steering element position for areference wavelength (e.g., where a peak occurs in the referencespectrum) is known as a reference data point. The controller 740interpolates reflective beam steering element positions and theircorresponding wavelengths using algorithms stored in the memory 745 andthe reference data points. The controller 740 may then update its lookuptable with revised calibration data.

Additionally, in some embodiments, the resolution of the tunable filter730 may be increased using signal processing techniques described abovewith reference to FIG. 6. For example, controller 740 may be configuredto convert the electronic output of the photodetector 760 into a digitalsignal and perform signal processing techniques on the digital signal.This embodiment may be useful in cases where the reference wavelengthsare closely spaced.

Memory 745 also stores programmable parameters for monitoring opticalsignals received by OCM device 170 through one or more inputs, and theoptical power of different optical channels as measured based on signalsfrom photodetectors 760. Optical channel monitoring is further describedin U.S. patent application Ser. No. 13/226,275.

In alternate embodiments, the source filter 720 is an etalon filter andgenerates a reference spectrum that includes one or more peaks withinthe operating wavelength range. In this case, the output of sourcefilter 720 is coupled to the tunable filter 730 via an auxiliary port(not shown). In some embodiments, the source filter 720 may beconfigured such that the peaks of the reference spectrum correspond tolocations on an International Telecommunication Union (“ITU”) grid.

Note, that FIG. 7 includes a light source 710. An alternative embodimentuses the optical signals from an output port of a tunable filter as alight source. FIG. 8 is a block diagram of an example OCM device 800configured to self-calibrate using an optical signal an output port of atunable filter to generate a reference spectrum. The OCM device 800includes a tunable filter 730, photodetector 760, a splitter 810, anetalon filter 820, a controller 830, and photodetector 840. While only asingle input and a single output (and corresponding photodetector 760)are shown, it should be understood that the invention is applicable toan OCM device with any number of optical input ports and any number ofphotodetectors 760.

The output of the tunable filter 730 is split via splitter 810. One partof the split output is provided to a photodetector 760 for monitoring bythe OCM device 800. The other part of the split output is provided tothe etalon filter 820 which treats it as a light source to generate areference spectrum. At any given time, the input spectrum to the etalonfilter 820 contains only a narrow range of wavelengths (e.g., thosepassing through the tunable filter 730), thus there is only onecorresponding peak at the output of the etalon filter 820. Additionally,because of the periodicity of the etalon filter 820, there is the dangerof mistaking a channel n for n−1, or vice versa. This may be avoided byensuring that the tunable filter wavelength is stable within +/−25 GHz.To ensure that the channel light is not rejected by the etalon filter820, the peaks of the etalon transmission spectrum may be aligned to theITU grid, as shown in FIG. 8. In this embodiment, the etalon filter 820may have a 50 GHz or a 100 GHz free spectral range. The photodetector840 detects the reference spectrum and provides an electronic form ofthe reference spectrum to the controller 830. In other embodiments, theetalon transmission spectrum may be aligned to some other frequencylocations.

The controller 830 is configured to execute instructions that calibratethe tunable filter 730 using the reference spectrum. The calibration mayoccur continuously, periodically, upon receipt of a calibration command,or some combination thereof. Because the reference spectrum generatespeaks at known wavelengths the controller 830 is able to calibrate thetunable filter 730. For example, memory 745 may contain a lookup tablethat maps wavelength (or frequency) to a reflective beam steeringelement's position (e.g., mirror 16). During a calibration process, thetunable filter 730 may scan across a range of wavelengths until at leastone peak is present in the reference spectrum that is detected via aphotodetector 760. The controller 830 records the reflective beamsteering element positions in memory 740 (e.g., determines referencedata point for each detected peak in the reference spectrum). Thecontroller 830 calculates the reflective beam steering element positionsand their corresponding wavelengths using algorithms stored in thememory 740 and the reference data points. Using a larger number ofreference data points in the calibration process results in a moreaccurate calibration data. The controller 830 may then update its lookuptable with revised calibration data. It is also worth noting that,considering that the OCM device 800 reports the wavelengths of thechannels that are present, and that the reference wavelengths aregenerated by those channels, the wavelength reporting accuracy is notaffected by the number of channels that are present.

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the disclosure but merely asillustrating different examples and aspects of the disclosure. It shouldbe appreciated that the scope of the disclosure includes otherembodiments not discussed in detail above. For example, in someembodiments FIG. 7 may be modified such that the Source Filter 720 maybe connected to the Tunable Filter 730 output through a splitter.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

In the claims, reference to an element in the singular is not intendedto mean “one and only one” unless explicitly stated, but rather is meantto mean “one or more.” In addition, it is not necessary for a device ormethod to address every problem that is solvable by differentembodiments of the invention in order to be encompassed by the claims.

Each controller (e.g., controller 740 and controller 830) may beimplemented in computer hardware, firmware, software, and/orcombinations thereof. Each computer program can be implemented in ahigh-level procedural or object-oriented programming language, or inassembly or machine language if desired; and in any case, the languagecan be a compiled or interpreted language. Suitable processors include,by way of example, both general and special purpose microprocessors.Generally, a processor will receive instructions and data from aread-only memory and/or a random access memory. Any of the foregoing canbe supplemented by, or incorporated in, ASICs (application-specificintegrated circuits) and other forms of hardware. Some of the methodsperformed by the computer may be implemented using computer-readableinstructions that can be stored on a tangible non-transitorycomputer-readable medium, such as a semiconductor memory.

What is claimed is:
 1. A tunable multiport optical filter comprising: a linear array of optical input ports extending along an x-direction, the optical input ports positioned on a regular grid in the x-direction, the regular grid characterized by a pitch; an optical output port positioned collinearly with the optical input ports but off the regular grid; a light dispersion element positioned to receive an optical signal from one of the optical input ports and to disperse the optical signal along a y-direction that is not parallel to the x-direction; and a reflective beam steering element positioned to receive the dispersed optical signal from the light dispersion element and to reflect the dispersed optical signal back to the light dispersion element, the reflective beam steering element controllable such that any wavelength portion of the dispersed optical signal from any optical input port can be reflected via the light dispersion element to couple to the optical output port.
 2. The tunable multiport optical filter of claim 1, wherein the linear array of optical input ports is symmetric with respect to the optical output port.
 3. The tunable multiport optical filter of claim 1, wherein the optical output port is positioned outside of the array of optical input ports.
 4. The tunable multiport optical filter of claim 1, wherein the reflective beam steering element is further controllable to couple some of the linear array of optical ports to others of the linear array of optical ports.
 5. The tunable multiport optical filter of claim 1, further comprising: a photodetector and analog to digital converter coupled to the optical output port and configured to convert the received optical signal to a digital signal; and a signal processor configured to apply a deconvolution to the digital signal.
 6. The tunable multiport optical filter of claim 1, wherein the optical signal is a wavelength-division multiplexed signal containing multiple channels at different wavelengths, and the reflective beam steering element is controllable such that any channel of the dispersed optical signal can be reflected via the light dispersion element to couple to the optical output port.
 7. The tunable multiport optical filter of claim 6, wherein the wavelength-division multiplexed signal complies with an ITU grid.
 8. A self-calibrating optical channel monitoring (OCM) device comprising: a tunable multiport optical filter comprising: a plurality of optical input ports, at least one optical output port, a light dispersion element positioned to receive and disperse an optical signal from the optical input ports, and a reflective beam steering element that is controllable to deflect any wavelength portion of the dispersed optical signal from any optical input port to the optical output port; and a photodetector coupled to the optical output port; a reference spectrum source that produces a reference spectrum that can be coupled to the tunable filter; and a controller coupled to the reference spectrum source, the tunable filter and the photodetector; the controller configured to: control production and/or coupling of the reference spectrum to the tunable filter, and calibrate the tunable filter based on a response of the photodetector when the reference spectrum is coupled to the tunable filter.
 9. The OCM device of claim 8, wherein the optical signal has an operating wavelength range, the reference spectrum includes two reference peaks, and the controller calibrates the tunable filter based on the response of the photodetector to the two reference peaks.
 10. The OCM device of claim 9, wherein the two reference peaks lie outside the operating wavelength range.
 11. The OCM device of claim 8, wherein the reference spectrum source comprises a light source coupled to a source filter.
 12. The OCM device of claim 11, wherein the filter is a dual band pass filter.
 13. The OCM device of claim 11, wherein the filter is an etalon filter.
 14. The OCM device of claim 8, wherein the reference spectrum includes a plurality of reference wavelengths, and the controller is configured to determine a plurality of reference data points that map a plurality of reference wavelengths to corresponding positions of the beam steering element.
 15. The OCM device of claim 11, wherein the optical signal is a wavelength-division multiplexed signal containing multiple channels at different wavelengths, and the reflective beam steering element is controllable such that any channel of the dispersed optical signal can be reflected via the light dispersion element to couple to the optical output port.
 16. The OCM device of claim 15, wherein the wavelength-division multiplexed signal complies with an ITU grid.
 17. The OCM device of claim 8, wherein the at least one output port is a detector.
 18. A self-calibrating optical channel monitoring (OCM) device comprising: a tunable multiport optical filter comprising: a plurality of optical input ports, at least one optical output port, a light dispersion element positioned to receive and disperse an optical signal from the optical input ports, and a reflective beam steering element that is controllable to deflect any wavelength portion of the dispersed optical signal from any optical input port to the optical output port; and a splitter coupled to the optical output port and configured to split an output of the tunable filter into at least two portions; a first photodetector coupled to detect one of the split portions; an etalon filter coupled to receive another of the split portions and configured to generate a reference spectrum therefrom, wherein the reference spectrum contains at least one reference peak; a second photodetector coupled to an output of the etalon filter; and a controller coupled to the second photodetector and tunable filter, the controller configured to calibrate the tunable filter based on a response of the second photodetector:
 19. The OCM device of claim 18, wherein the reference spectrum is based on an ITU grid.
 20. The OCM device of claim 18, wherein the reference spectrum includes a plurality of peaks.
 21. The OCM device of claim 20, wherein the controller is configured to determine a plurality of reference data points that map positions of the beam steering element to wavelengths that correspond to peaks on the ITU grid.
 22. The OCM device of claim 19, wherein the optical signal is a wavelength-division multiplexed signal containing multiple channels at different wavelengths, and the reflective beam steering element is controllable such that any channel of the dispersed optical signal can be reflected via the light dispersion element to couple to the optical output port.
 23. A tunable multiport optical filter comprising: a two-dimensional array of optical ports, including both optical input ports and corresponding optical output ports, the two-dimensional array extending along both an x-direction and a y-direction that is not parallel to the x-direction; a light dispersion element positioned to receive optical signals from the optical input ports and to disperse the optical signals along the y-direction; and a reflective beam steering element positioned to receive the dispersed optical signals from the light dispersion element and to reflect the dispersed optical signal back to the light dispersion element, the reflective beam steering element controllable such that any wavelength portion of the dispersed optical signals can be reflected via the light dispersion element to couple to the corresponding optical output ports without significant coupling to other optical ports.
 24. The tunable multiport optical filter of claim 23, wherein the two-dimensional array of optical ports comprises at least two one-dimensional subarrays of optical ports, each of the two one-dimensional subarrays extending along the x-direction and characterized by a pitch, wherein the two one-dimensional subarrays are offset in the x-direction by a fraction of the pitch.
 25. The tunable multiport optical filter of claim 24, wherein there are two one-dimensional subarrays and the two subarrays are offset in the x-direction by 0.5×pitch.
 26. The tunable multiport optical filter of claim 24, wherein there are 2 m one-dimensional subarrays with m>1, and the subarrays are offset in the x-direction by 1/(2 m)×pitch.
 27. The tunable multiport optical filter of claim 23, wherein the two-dimensional array of optical ports is symmetric about an (x,y) origin, and optical ports located at (x,y) and (−x-y) form pairs of optical input port and corresponding optical output port.
 28. The tunable multiport optical filter of claim 23, wherein the optical signal is a wavelength-division multiplexed signal containing multiple channels at different wavelengths, and the reflective beam steering element is controllable such that any channel of the dispersed optical signal can be reflected via the light dispersion element to couple to the optical output port.
 29. The tunable multiport optical filter of claim 28, wherein the wavelength-division multiplexed signal complies with an ITU grid. 